In order to produce ethylene and other short-chain olefins, some hydrocarbon oil fractions are cracked thermally in tubular metal reactors. The resulting cracked gases are suddenly cooled in heat exchangers which operate by supplying water and steam under pressure.
The tubular reactors used are preferably manufactured from chromium- and nickel-rich steels, while the heat exchangers, which are subjected to less obvious stresses, are composed of carbon steels or weakly alloyed steels. This same type of equipment is also encountered in producing other organic compounds, such as vinyl chloride by pyrolysis of 1,2-dichloroethane.
The effectiveness of these reactors and heat exchangers made of steel depends on their resistance to the formation of a deposit of coke on their internal walls in contact with the hydrocarbon to be cracked. Not only is this deposit harmful to heat transfer but it reduces the effective cross section of the tube. The thickness of this coke deposit becomes such that the unit has to be halted and subjected to expensive cleaning operations. In the majority of cases, the coke deposit is removed by high-temperature gasification with a mixture of steam and air which converts the coke to carbon oxides and re-establishes the starting characteristics of the cracking tube. When the deposit is produced in the heat exchangers, it is not possible structurally to carry out an in-line decoking by gasification as the maximum temperatures acceptable are too low to allow this reaction. Dismantling and manual decoking are necessary, which is a difficult and expensive operation.
Hydrocarbon cracking units, such as steam crackers, are thus frequently shut down in order to be subjected to decoking cycles (after operating for 20 to 60 days). Furthermore, the oxidizing decoking treatment results in an increase in the catalytic activity of the metal cracking surface, which increases the rate of formation of coke. Thus, with the increase in the number of decokings undergone by the unit, the operating time decreases and the annual number of decoking operations increases. This long-term effect is injurious technically and economically since the maintenance costs become more and more burdensome with the age of the unit for a lower annual operating rate.
This is the reason why numerous efforts have been made for many years to find solutions which prevent the rapid coking of the internal metal walls of such units (cracking tubes and heat exchangers).
It is standard practice in the industrial production of ethylene to inject, with the hydrocarbon feedstock, relatively small amounts of sulphur-comprising products, such as dimethyl sulphide (DMS) or dimethyl disulphide (DMDS), in order to reduce the formation of coke. It is commonly accepted by the person skilled in the art that the sulphur passivates the active metal sites of the surface of the steam cracking tubes which are known to catalyze the formation of coke. Moreover, these sulphur-comprising compounds are known to also reduce the formation of carbon monoxide (CO) formed by the reaction of hydrocarbons or coke with steam, by passivating in the same way the active metal sites of the surface of the tubes. In fact, it is also important to minimize the amounts of CO produced in order to ensure correct operation of the separation and purification units downstream of the steam cracker.
These technical solutions and many others are described in the literature; mention may more particularly be made of the following:
U.S. Pat. No. 4,116,812 describes a process for inhibiting coke using organosulphur additives of dithiolthione type at temperatures of 260 and 815° C.
U.S. Pat. No. 5,463,159 describes a method for treating the furnaces used in the production of ethylene with compounds which generate hydrogen sulphide (H2S) in order to reduce the formation of coke. The preferred H2S-generating compound is dimethyl sulphide (DMS).
U.S. Pat. No. 5,616,236 discloses a method for inhibiting coke and CO by treatment of the tubes of the steam cracker with sulphur-comprising compounds in the presence of hydrogen, the sulphur-comprising compounds being of sulphide or disulphide type, DMS and DMDS being particularly preferred.
U.S. Pat. No. 5,954,943 describes a method for reducing the deposition of coke in cracking furnaces using a mixture of sulphur-comprising and phosphorus-comprising compounds with an S/O molar ratio of greater than or equal to 5.
EP 976 726 A1, which is concerned with DMDS possessing a masked odour, indicates in [0002] that it can be used as additive for charging to a steam cracker.
WO 2005/111175 describes a process for thermal cracking of hydrocarbon compounds and the use in the mixture as coke inhibitor of a mixture of organic disulphides with a carbon number of between 2 and 4 known as sulphur oil or disulphide oil (DSO). The preferred mixture is a mixture of alkyl disulphides where the alkyl groups are methyl and ethyl and where the typical sulphur content is 60%.
Coke inhibitors other than sulphur-comprising compounds are also known. Mention may be made for example of U.S. Pat. Nos. 4,507,196, 4,511,405, 4,552,643, 4,613,372, 4,666,583, 4,686,201, 4,687,567, 4,804,487 and 5,015,358, which teach that the metals Sn, Ti, Sb, Ga, Ge, Si, In, Al, Cu, P and Cr, and their organic and inorganic derivatives, individually or as a mixture, act as coke inhibitors during the pyrolysis of the hydrocarbons.